**UNIT 2**

# Pharmaceutical Calculations:

**Weight and measures**

There are 2 systems of weight and measures,

**Imperial system**

**Metric system**

**Imperial system:-**

**Avoirdupois system**– Pound is the standard unit for weighting all measures of all mass are derived from it.(1 pound = 5760 grains)

**Apothecaries system (Troy stem)** – Grain is the standard unit in this system

**Metric system:-**

**Measurement of weight** – Kilogram is the standard unit of measurement(1 kg= 100 gm)

**Measurement of capacity** – liter is the standard unit of measurement (1 L = 100 ml)

###### Pharmaceutical Calculations Notes

**Calculations involving % solutions:-**

Calculate the amt. of 95% alcohol required to prepare 400 ml of 45% alcohol.

Vol. req. = 400 ml, % alcohol required= 45%, % alcohol used= 95%

Vol. of strong alcohol to be used = vol. required * % required / % used

= 400*45/95

=184.47 ml

### Alligation Method:-

When calculations involve mixing of 2 similar preparations of diff. strength, to produce a preparation of intermediate strength, the alligation method is used.

Calculate the volume of 95 % alcohol required to prepare 600 ml of 70 % alcohol

Vol. required = 600 ml, % alcohol required = 70, % alcohol used = 95

70 parts of 95% alcohol & 25 parts water will produce the required % alcohol

Qty of 95% alcohol req. = 600*70/ 95 = 422.10 ml

Qty of water req. = 600*25/95 = 157.90 ml

**Proof spirit Calculation:-**

The strength of alcohol is calculated in proof degree. As per Indian standards 100% proof spirit = 57% v/v ethyl alcohol

The value of more than 57 % = overproof spirit

The value less than 57 % = under a proof spirit

Calculate the proof strength of 95% v/v alcohol.

Proof strength= % strength of alcohol* 1.753-100

=95*1.753-100

= 66.53° O.P

**Isotonic solution:-**

The solutions having same osmotic pressure or equal solute concentration is called an isotonic solution.

Freezing point:-

% w/v of adjusting substance needed = 0.52-PSM*a / b

Where, PSM= percentage strength of the measurement

A= freezing point of the unadjusted solution

B =freezing point of 1% w/v sol. of adjusted solution

**Molecular weight:-**

% w/v of adjusting substance needed = 0.03M/N

Where M= gram molecular weight

N= no. of ions into which substance is ionized

###### Pharmaceutical Calculations Notes

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